Alarm just inside marshy area (8)
I believe the answer is:
frighten
'alarm' is the definition.
(thesaurus)
'just inside marshy area' is the wordplay.
'just' becomes 'right' ('right' can be similar in meaning to 'just').
'inside' means one lot of letters goes inside another.
'marshy area' becomes 'fen' (I've seen this before).
'right' placed within 'fen' is 'FRIGHTEN'.
(Other definitions for frighten that I've seen before include "Cause fear" , "Put the wind up" , "Cow" , "Terrify" , "Shock" .)